1. Field of the Invention
The present invention relates generally to lasers, and more particularly to diode-pumped Q-switched miniature lasers.
2. Prior Art
Basic theories of pulsed mode laser operation were introduced by W. G. Wagner and B. A. Lengyel in "Evolution of the Giant Pulse in a Laser," Journal of Applied Physics vol. 34 No. 7 pp. 2040-2045 (1963). U.S. Pat. No. 3,613,024 by Geusic describes a solid-state Nd:YAIG laser cavity in which, to build up pulses, the internal beam resonance is interrupted temporarily by action of an acousto-optic Q-switch. The Q-switch has normal-incidence surfaces with anti-reflection coatings to reduce optical losses. Laser cavities can be pulsed efficiently by Q-switches using acoustic waves.
Lasers in many applications require operation with polarized light. Some solid-state crystalline laser gain materials inherently promote polarized light operation. Other conventional solid-state laser cavities as shown in FIG. 1 are polarized by passing light in beam B along optical axis OA through an acousto-optic Q-switch 10 having Brewster-angled surfaces 12 and 14. The Brewster-angled surfaces polarize light without exacting losses in one dimension of beam B as indicated by arrow 11 in the plane of FIG. 1. Optics which minimize reflection losses by Brewster-angled surfaces are generally more durable and less expensive than the alternative way of reducing reflection losses by using anti-reflection coatings.
Exploitation of Q-switches 10 having Brewster-angled surfaces 12, 14 is complicated, however, by the fact that upon penetrating surfaces non-normally, such as Brewster angled surfaces of Q-switches, light rays in diverging beams encounter effectively different path lengths in the sagittal and tangential planes. Different path lengths cause astigmatism in divergent beams, i.e. beam shapes become aberrated. This typically reduces laser energy extraction efficiency and power output. Various effects on resonant modes of cavities with non-normal incidence optical elements were analyzed by H. Kogelnick and T. Li in "Laser Beams and Resonators," Proceedings of the IEEE vol. 54 pp. 1312-1329 (Oct. 1966), and further by G. A. Massey and A. E. Siegman in "Reflection and Refraction of Gaussian Beams at-Tilted Ellipsoidal Surfaces," Applied Optics vol. 8 pp. 975-978 (1969).
Brewster-angled surface-induced astigmatism increases both with beam divergence and path length in traversing obliquely-intercepted media. Aberrations are worse for beams either diverging more widely and/or traveling longer distances through an oblique Q-switch. Conversely, aberrations are less severe for beams diverging less widely or traveling shorter distances. In a relatively large, continuously lamp-pumped one-meter long Nd:YAG laser cavity apertured for the 1.064 um wavelength lasing in the TEM.sub.00 mode, internal beams usually have thick (1 mm) diameter waists and correspondingly narrow (e.g., 1.4 milliradian) far-field divergences. Such a narrow divergence beam is only negligibly aberrated by traversing a 5 cm long Brewster-angled Q-switch 10, and emerges with less than 1% circular distortion, permitting the laser to run essentially diffraction-limited.
Contrastingly, in small cavities pumped by laser diodes, internal Gaussian beams commonly have smaller 100 to 300 micron diameter waists along with broader 5 to 15 milliradian far-field divergences. L.J. Rosenkrantz describes a laser with a diode end-pumping an Nd:YAG gain medium in "GaAs Diode-Pumped Nd:YAG Laser," J. App. Phys. vol. 43 No. 11 pp. 4603-4605 (Nov. 1972). K. Iwamoto, I. Hino, S. Matsumoto and K. Inoue report extended area diodes used for end-pumping cavities in "Room Temperature CW Operated Superluminescent Diodes for Optical Pumping of Nd:YAG Laser," Japanese Journal of Applied Physics vol. 15 No. 11 pp. 2191-2194 (Nov. 1976).
Lasers combining diode-pumping with Q-switching were proposed by M. Ross in "YAG Laser Operation by Semiconductor Laser Pumping," Proceedings of the IEEE pp. 196-197 (Feb. 1968). Later demonstrations were reported by G.M. Zverev, I.I. Kuratev, and A.V. Shestakov in "Solid State Microlasers Based on Crystals with a High Concentration of Neodymium Ions," Bulletin of the Academy of the Sciences of the USSR, Physical Sciences, vol. 46 No. 8 pp. 1561-1566, English trans. pp. 108-112 (1982) Early diode-pumped Q-switched solid-state laser reports are summarized by I. I. Kuratev in "Solid State Lasers with Semiconductor Pumping," Bulletin of the Academy of the Sciences of the USSR, Physical Sciences, vol. 48 No. 8 pp. 1564-1572, English trans. pp. 104-112 (1984).
In relatively small cavities, astigmatism induced by a Brewster-angled Q-switch 10 can diminish output power by as much as half. With a Brewster-angled Q-switch 10 which is 5 cm long, a near-concentric cavity must be less than 10 cm long to form a small (100 to 300 um) diameter mode volume. Near-confocal cavities are less sensitive to astigmatism influences from optical interfaces at Brewster angles.
Miniature diode-pumped Q-switched laser cavities 20 as shown in FIG. 2 conventionally use acousto-optic Q-switches 22 with normal-incidence opposite end surfaces 24 and 26. A laser diode (not shown) pumps light from the left in a beam along optical axis OA, which enters through a back mirror into cavity 20. A back mirror may be either an optional separate element 28 or a coating 30 on the end surface of gain material 32. In either case, light entering along axis OA traverses gain material 32 and emerges from normal incidence surface 34. Light along axis OA next penetrates Brewster angled polarizing plate 36, which is typically so thin (0.5 to 1.0 mm) that it induces only negligible aberrations in beam B. Brewster plates 36 are suitably made from materials such as industry standard fused silica. To avoid output beam aberrations which could be caused by-use of astigmatic optical elements, FIG. 2 type miniature lasers have previously refrained from incorporating FIG. 1 type Brewster-angled surface Q-switches 10.
G.T. Maker and A.I. Ferguson disclose a polarizing structure with a gain material having one Brewster-angled surface, but which has coatings on various surfaces, in "Mode Locking and Q-Switching of a Diode Laser Pumped Neodymium Doped Yttrium Lithium Fluoride Laser," Applied Physics Letters vol. 54 No. 5 pp. 403-405 (1989).
Diode-pumped Q-switched miniature lasers remain in need of ways to polarize cavities without incurring aberrations or output power losses.